A system and method for determining the geolocation of autonomous mobile appliances emitting analog waveforms is disclosed. More specifically, the inventive system and method is used to geolocate FM analog signals such as those used in the AMPS cellular radio air standard by using a time difference of arrival (“TDOA”) approach.
There is currently much focus on technology for performing geolocation of cellular phones and other mobile appliances. This has been largely motivated by an FCC mandate requiring the location of wireless users to be provided to the Public Service Answering Point (“PSAP”) when making an emergency 911 (“E911”) call or other 911-related transmission. There are currently many systems and patents that deal with the source location of wireless radio frequency emitters. To a great extent, these patents simply suggest the idea of location of a transmitter source by some means, which is often impractical to implement for an application such as an E911 event. Very often, factors such as cost, complexity, and required computation horsepower are not fully considered.
Prior art systems usually deal with the source location of either analog or digital signals. The analog signals referred to herein are those signals which are not encoded for the purpose of transmitting digital information, but rather analog waveforms through means such as frequency modulation (“FM”). Digital signals, on the other hand, are those that employ some form of information bit to pulse shaped symbol encoding for the purpose of transmitting digital information, theoretically allowing loss-less transmission of data. The pulse-shaping and modulation schemes used in digital waveforms can be exploited to reduce the cost and resources required for a location system, in a manner that is not available in analog location systems. The present inventive system and method focuses on a location method used specifically for FM analog signals such as those used in the AMPS cellular radio air standard. It is to be understood that the present invention is not limited to a particular type of emitter of FM analog signals, such as AMPS or the AMPS air standard. Rather, the present invention is applicable to any system emitting FM analog signals. The motivation for the present invention has been driven by the requirements to design a geolocation system that reduces system cost and latency while simultaneously maximizing geolocation accuracy.
Radio location systems and methods can be grouped into two major classes. The first class of radio location systems and methods uses the arrival angle of a radio frequency (“RF”) signal at an antenna array to determine a line of bearing from the array to the emitter of the RF signal. For example, a mobile radio transmits a signal which is received by multiple base stations in separate geographic locations. Each base station has an antenna array which measures the radio wave phase difference at different antenna elements in the array. An angle of arrival of the mobile radio's signal is calculated and a line of bearing to the mobile radio is determined. By obtaining multiple lines of bearing from multiple antenna arrays, the intersection of the lines of bearing provides the geolocation of the mobile radio.
The second class of radio location systems and methods uses the time differences of arrival from geographically separated sensors in order to estimate the emitter location through triangulation techniques. For example, a mobile radio transmits a signal which is received by multiple base stations in separate geographic locations. Each base station conducts time difference of arrival (“TDOA”) measurements of the received signal and the TDOA measurements are used to determine the location the mobile radio using conventional positioning algorithms. A global positioning system (“GPS”) or other time standard is typically used to provide a common time reference among the base stations and the mobile radio. Typically, a TDOA-based analog signal location system comprises multiple geographically separated sensors, referred to herein as Wireless Location Sensors (“WLS”) that are connected and controlled through some communication means such as telephone lines of high speed data communication lines such as trunked DS0 or ISDN. All of the WLSs are controlled by a central processing facility, referred to herein as a Geolocation Control Server (“GCS”) that tasks the sensors to simultaneously capture signals transmitted by a particular mobile radio, also referred to herein as a “mobile station”. The captured signals are then sent to the GCS via telephone lines or high speed data communication lines, along with the precise time measurement of when the signals were captured. The location of the mobile station is then calculated through cross-correlations of the signals received at the GCS from the WLSs—the peaks of the captured signals reveal an estimate of the time differences of arrival of the mobile station's signals at the various WLSs. The time differences of arrival are geometrically interpreted as branches of hyperbolic surfaces that intersect at the location of the mobile station.
Each of the WLSs includes a precise time source, such as those derived from a GPS disciplined oscillator or other time standard, a radio frequency receiver, digitizing circuitry, a digital processor, and other standard circuitry such as an analog to digital (“A/D”) converter, all of which operate to capture, store, and manipulate the received signals. However, in order to obtain an acceptable level of accuracy when performing a geolocation evolution, it is highly desirable to receive and process as much data as possible from the received signal. The more data that is used to determine the geolocation of the mobile station, the greater the accuracy of the geolocation estimate. One processing limitation that must be taken into account is the sampling rate. Due to the physical limitations of Nyquist sampled real signals, the sampling rate must be set to at least twice the rate of the highest frequency component of the received signal. Additionally, the level of quantization used at the analog to digital converter stage must be sufficient to capture a broad range of signal levels without significant distortion. All this data must be sent to the GCS, which requires that the data be sent over telephone lines of high speed data communication lines. Since the WLSs are located in geographically different locations, the data used for the geolocation calculation must be sent over telephone or high speed data communication lines from at least one WLS.
An example of the amount of data that must be sent from a WLS to the GCS, and the amount of time to send that data, consider a one second AMPS analog waveform received at a WLS and used to locate the mobile station to be geolocated. For the AMPS analog waveform, the baseband double-sided bandwidth is 30 KHz. Further consider that 16-bit A/D converters are used on each of two receive channels to provide approximately 96 dB of dynamic range and that the sampling rate is 40,000 complex samples/sec. Additionally, the data link between the WLS and the GCS is a DS0 high speed digital data communication line with a data transport rate of 64,000 bits per second (“bps”). Note that 40 KHz is a practical over-sampling rate which will allow sufficient excess bandwidth for filtering of adjacent signals and anti-aliasing, as is known in the art. In order to capture one full second of signal data for sufficient cross-correlating of the signals received at the GCS from the WLSs, the required amount of data in bits to be transmitted from a WLS to the GCS can be calculated as follows:             (              40        ,        000        ⁢                  samples          sec                    )        ×          (              1        ⁢                                   ⁢        sec            )        ×          (              2        ⁢                                   ⁢        channels            )        ×          (              16        ⁢                  bits          sample                    )        ×          (              2        ⁢                                   ⁢        samples            )        =      2.56    ⁢                   ⁢    Mbits  
The 2.56 Mbits of data is the total for two signals to be cross-correlated. Using a 64 kbps data rate for a DS0 high speed digital data communication line, the time to transfer the 2.56 Mbits of data can be calculated as follows:             (              2.56        ⁢                                   ⁢        Mbits            )        ÷          (              64        ⁢                  Kbits          sec                    )        =      40    ⁢                   ⁢    sec  
As shown above, the time it takes to transfer a sufficient amount of data from an AMPS signal from a WLS to the GCS in order to accurately geolocate the mobile station is 40 seconds. This is clearly an unacceptable amount of time for applications where a high throughput of location estimates is required. Certain prior art systems attempt to overcome the data transfer problem by either limiting the amount of data sampled or by using excessively lossy data compression schemes. Thus, there is a need for a geolocation system and method for accurately geolocating a mobile station in a practical and efficient manner.
The present inventive system and method increases the speed of location estimates without sacrificing accuracy of the geolocation estimate. The inventive system and method does not use smaller sample sizes or excessively lossy data compression schemes. The inventive system comprises multiple WLSs that are typically co-located with the base stations of the mobile station's communication network, and a centrally-located GCS. The WLSs operate in one of two modes, a primary mode and a secondary mode, the operation of each will be described in detail below. Generally, once the system receives a request to locate a wireless user, or mobile station, each WLS operates in primary mode to initiate the geolocation evolution and send information regarding the signal received from the mobile station to the GCS. Upon receipt of the signals from the multiple WLSs, the GCS selects one WLS to be the primary WLS, the significance of which will be discussed later. The primary WLS continues to operate as before as well as operating in the secondary mode. The remaining WLSs switch to and operate in the secondary mode. The details of the operation of the system is disclosed below.
Accordingly, it is an object of the present invention to provide a novel system and method of geolocating a mobile station transmitting FM analog signals such as those used in the AMPS cellular radio air standard from a plurality of WLSs located in geographically spaced-apart locations.
It is another object of the present invention to provide a novel system and method for geolocating a mobile station transmitting an AMPS analog signal by reducing the amount of data to be transmitted across data communication lines.
It is yet another object of the present invention to provide a novel system and method for geolocating a mobile station transmitting and AMPS analog signal by combining multiple signals received at a WLS to a single channel, demodulating the single channel, and compressing the single channel by use of a Fourier transform circuit.
It is still another object of the present invention to provide a novel system and method for efficiently and accurately geolocating a mobile station by parallel processing the received signal data at the WLSs rather than at the GCS.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.